Spinal fixation devices are used in orthopedic surgery to align and/or fix a desired relationship between adjacent vertebral bodies. Such devices typically include a spinal fixation element, such as a relatively rigid fixation rod, that is coupled to adjacent vertebrae by attaching the element to various anchoring devices, such as hooks, bolts, wires, or screws. Alternatively, two rods can be disposed on the lateral or anterior surface of the vertebral body in a substantially parallel relationship. The fixation rods can have a predetermined contour that has been designed according to the properties of the target implantation site and, once installed, the rods hold the vertebrae in a desired spatial relationship, either until desired healing or spinal fusion has taken place, or for some longer period of time.
Spinal fixation devices can be anchored to specific portions of the vertebra. Since each vertebra varies in shape and size, a variety of anchoring devices have been developed to facilitate engagement of a particular portion of the bone. Pedicle screw assemblies, for example, have a shape and size that is configured to engage pedicle bone. Such screws typically include a threaded shank that is adapted to be threaded into a vertebra, and a receiving member having a U-shaped slot for seating the fixation rod. The receiving member can be monoaxial and thus fixed relative to the threaded shank, or it can be polyaxial and thus movable relative to the threaded shank. Polyaxial screws can facilitate positioning of the fixation rod therein. Extension instruments are often coupled to the receiving member, especially in minimally invasive procedures, to provide a pathway through tissue to the receiving member. A set screw, plug, or similar type of closure mechanism, is used to lock the fixation rod into the receiving member of the pedicle screw.
While current spinal fixation systems have proven effective, difficulties have been encountered in mounting rods into the receiving member of various fixation devices. In particular, it can be difficult to align and seat the rod into the receiving portion of adjacent fixation devices due to the positioning and rigidity of the vertebra into which the fixation device is mounted. While polyaxial pedicle screws can facilitate positioning of the fixation rod within the receiving member, additional correction of spinal deformities is often required. For example, the alignment of multiple vertebral levels can require manipulation of the extension instruments at each level to achieve the desired results. During these manipulation steps, the polyaxial screw is converted into a monoaxial screw to allow the surgeon to grasp the extension instrument and thereby manipulate the vertebra coupled thereto. While closure mechanisms can provide the ability to intraoperatively lock the polyaxial feature of the screw independent of locking the fixation rod within the receiving member, the closure mechanism can only be utilized after the rod is positioned within the receiving member, as it is inserted in a top-down fashion to press the rod into the U-shaped slot. In an open procedure, the rod can often be placed within the receiving member prior to correction, and thus conversion of the polyaxial screw into a monoaxial screw can often be achieved using the closure mechanism. In a minimally invasive deformity correction procedure, however, surgeons often utilize a “rod second” approach. This means that the spine is provisionally corrected prior to placing the rod within the receiving member. In a “rod second” approach the closure mechanism cannot be utilized to convert the polyaxial screw into a monoaxial screw.
Accordingly, there is a need for devices and methods for intraoperatively and selectively converting a polyaxial screw into a monoaxial screw to facilitate deformity correction and other operations during spinal surgery.